1. Field of the Invention
The present invention relates to a brush motor provided with a magnetic sensor for detecting a change in a field magnet that occurs in correspondence with a change in a rotational position of a rotor core.
2. Description of the Related Art
A frequency generator (hereafter referred to as FG) is used in the detection of the rotational speed of a dc motor. In the case of a brushless motor, a configuration has been proposed in which a sensor such as an FG coil for detecting the magnetic field from a rotating permanent magnet is provided (JP-UM-B-4-48145). In a brush motor, however, since the permanent magnet does not rotate, a conventional technique disclosed in JP-UM-B-7-13420 has been proposed.
Namely, in this technique, a disk-like rotating member fitted on a rotor and having a plurality of pole teeth on its outer periphery is provided. In addition, an annular disk-like stator is provided in contact with a motor housing and has a plurality of pole teeth on its inner periphery disposed in such a manner as to mutually oppose the pole teeth of the rotating member so as to overlap with the plate-like pole teeth of the rotating member. A permanent magnet for generating magnetic fields is provided between the pole teeth of the rotating member and the pole teeth of the stator. Further, a frequency detecting coil is provided for detecting a change in the magnetic field occurring between the pole tooth of the rotating member and the pole tooth of the stator. Consequently, when the rotor rotates, the magnetic resistance between the pole tooth of the rotating member and the pole tooth of the stator changes. Accordingly, since a change in the magnetic field occurs due to this change in the magnetic resistance, that change is detected by the frequency detecting coil (this technique will be referred to as the first prior art).
In addition, in the prior art proposed in JP-UM-A-58-57284 and JP-UM-B-60-15431, a rotating magnet is provided which has been magnetized into a plurality of magnetic poles and rotates integrally with a motor shaft. A change in the magnetic field occurring due to the rotation of this rotating magnet is detected by using a detection coil (this technique will be referred to as the second prior art).
In addition, in the prior art proposed in JP-UM-A-61-205277, magnetic poles are formed by magnetization on the outer periphery of a coreless armature at fine pitches. Further, a magnetic sensor is provided for detecting the magnetic field produced by each magnetic pole formed by magnetization on the outer peripheral portion of the armature. When the armature rotates, an output of a frequency proportional to the rotational speed of the armature is sent from the magnetic sensor (this technique will be referred to as the third prior art).
In addition, a conventional technique has been proposed concerning a case in which a brush motor not having a means for generating an FG signal is used. In this configuration, as shown in FIG. 14, a turntable 94 for rotatively driving a DVD is fixed to a motor shaft of a brush motor 91 mounted on a chassis 92 of a DVD reproducing apparatus. Further, a reflecting member is attached to a lower surface 941 of the turntable 94, and a reflection-type photointerrupter 95 is provided on a base plate 93. An output of the photointerrupter 95 is subjected to waveform shaping and used as an FG signal. Namely, a means for generating the FG signal is provided outside the brush motor 91 (this technique will be referred to as the fourth prior art).
The above-described first prior art requires the disk-like rotating member having a plurality of pole teeth on its outer periphery, a stator having a plurality of pole teeth on its inner periphery, a permanent magnet for forming magnetic fields between the pole teeth of the rotating member and the pole teeth of the stator, and a frequency detecting coil. For this reason, in the case where the first prior art is used, a problem has occurred in that the structure of the motor becomes complex. With the second prior art as well, since a separately provided rotating magnet is required for constructing the FG, problems have occurred in that the structure of the motor becomes complex, and that the shape becomes large in size.
In addition, the third prior art is a technique which is limited to a motor having a coreless armature. In the motor configured with a rotor core, magnetization for forming the FG cannot be directly effected on the outer periphery of the armature.
In the fourth prior art, since it is necessary to provide the photointerrupter 95 outside the brush motor 91, it is necessary to secure a gap L9 for the photointerrupter 95. Also, the base plate 93 for mounting the photointerrupter 95 is required. Consequently, since the turntable 94 must be provided at a position spaced apart from the chassis 92 by the portion of a distance L8, the turntable 94 inevitably protrudes from the chassis 92. For this reason, it has been difficult to flatten the shape of the apparatus using the brush motor 91 (DVD reproducing apparatus). (It should be noted that an arrangement may be provided such that the lower surface 941 of the turntable 94 is magnetized in a circumferentially divided fashion, and a change in the magnetic field due to this magnetization is detected by using a Hall element. However, in this case as well, the portion of the height of the Hall element and the portion of the width of the substrate 93 cause hindrances in flattering the shape of the apparatus.)